A conventional oil well may have a substantially tubular casing extended from the earth's surface to a subterranean oil bearing formation, and a well tubing positioned inside the casing extended between a bottom sump region having accumulations of liquid crude and a recovery wellhead at the surface. In most instances, the casing is perforated in the region of the formation to allow liquid crude seepage into a sump within the casing, and pump means cooperating with the well tubing in the sump, when activated, may move the crude upwardly through the well tubing for recovery at the wellhead.
The subterranean formation may be comprised of a porous or highly fractured rock reservoir, having solid granules of sand or rock separated by crevices and other spaces; and the oil bearing crude is contained in the crevices and other spaces.
In stripper or low producing wells, the crude may be of low concentration, and/or of a high viscosity and low mobility, and/or in pressure equilibrium with the perforated well casing, to the extent that the crude does not flow freely via the crevices and other spaces from the formation to the well casing and into the sump.
To overcome this and recover additional crude from the well, several methods of secondary recovery have been proposed involving the communication with the formation via the perforated well casing.
One approach is to seal the top of the well casing and then inject an incompressible liquid, like water, into the annular interior space between the well casing and tubing, and then build up intense superatmospheric pressures in the well casing and surrounding formation, in an attempt to break up the rock reservoir and/or open the crevices and other spaces to thereby improve the permeability of the formation. Fine solid particulates may be added to the water, to be injected with the water, which become wedged in the formation crevices when under the intense pressures and effective further then to hold the crevices open when the pressures are released.
Another approach, without the well casing being sealed, is to flood water into the well casing, to sink relative to the liquid crude and float the crude to a more concentrated higher pool for recovery. Hot steam instead may be injected into the well casing, to thermally heat the formation and thereby reduce the viscosity of the crude, to free the crude from the formation for flow then to the well sump for recovery. Liquid hydrocarbons, or a solvent or surfactant, may also be injected into the well casing, alone or with a carrier fluid, to chemically release the crude from the formation or dissolve some of the restricting formation. In some instances, combustible fluids may be injected into the well to initiate and support combustion of the crude in the formation, thereby again thermally heating the crude to reduce its viscosity; and further after the fire is water quenched, the steam generates some superatmospheric pressures.
A somewhat different approach proposed has been to evacuate gases from the sealed well casing space, by means of a vacuum pump or blower connected at the wellhead. The subatmospheric pressures in the well casing establish pressure differentials between the well casing and the surrounding formation, effective at times to draw oil bearing gases and liquid crude in the formation toward the casing, to accumulate in the well sump.
Suitable controls, responsive to the liquid crude accumulating in the well sump, may be used to activate the oil lift pump to remove the crude from the well.